Fuel cells are well known and are commonly used to produce electrical current from reducing fluid fuel and oxygen containing oxidant reactant streams, to power various types of electrical apparatus. Known solid oxide fuel cells (“SOFC”) generate both electricity and heat by electrochemically combining a fluid reducing fuel and an oxidant across an ion conducting electrolyte. In a typical SOFC, the electrolyte is an ion conductive ceramic membrane sandwiched between an oxygen electrode (cathode) and a fuel electrode (anode). Molecular oxygen, such as from the atmosphere, reacts with electrons at the cathode electrode to form oxygen ions, which are conducted through the ceramic membrane electrolyte to the anode electrode. The oxygen ions combine with a reducing fuel such as a mixture of hydrogen and carbon monoxide to form water and carbon dioxide while producing heat and releasing electrons to flow from the anode electrode through an electrical circuit to return to the cathode electrode.
Solid oxide fuel cells have many benefits and some limitations. For example, normal operating temperatures are very high, often in excess of 700° C., which favors stationary power plants operating in a near steady-state mode to minimize deleterious effects of thermal cycling as the fuel cell is started up and shut down. Solid oxide fuel cell based power plants nonetheless are designed to sustain a number of start-stop cycles throughout their operational terms such as for initial start-up, regular maintenance, repairs, etc. Additionally, most such solid oxide fuel cell power plants operate on a supply of natural gas that has to be processed to eliminate unacceptable components and to produce high quality, hydrogen-rich fuel. Such fuel processing systems typically include at least a desulfurizer, a reformer (such as a catalytic, steam reformer, and other reformers well-known in the art) and possibly a shift reactor, and a variety of heat exchangers configured to have the components of the fuel processing system operate at optimal temperatures.
A substantial problem associated with starting up solid oxide fuel cells with fuel processing systems is formation of soot, or coking of the fuel reactant stream as it passes through the fuel processing system at relatively low start-up temperatures. Many efforts have been undertaking to minimize problems associated with efficient heating of solid oxide fuel cell power plant components during start-up. For example, U.S. Pat. No. 7,410,016 that issued on Aug. 12, 2008 to Schumann et al. discloses use of a combustion chamber upstream of a catalytic reformer that ignites a portion of a fuel/air mixture to provide a hot combustion exhaust that is fed through fuel processing components to rapidly bring the components up to an operating temperature.
Additionally, U.S. Pat. No. 7,588,849 that issued on Sept. 15, 2009 to Haltiner, Jr. et al. discloses use of a tail gas combustor to direct flow of heated exhaust stream through jacket spaces surrounding fuel cells in a solid oxide fuel cell stack to rapidly heat up the fuel cells during a start-up procedure. More recently, U.S. Pat. No. 7,645,532 that issued on Jan. 12, 2010 to Weiss et al. discloses use of a reformate combustor between a reformer and a fuel cell stack so that partially burned reformate is passed through the anode chambers of the cell. Weiss et al. includes structures that thermodynamically decouple the fuel cell stack from components of the fuel processing system so that each may operate at differing optimal temperatures.
While these and many other disclosures seek to produce a rapid start-up procedure for a solid oxide fuel cell power plant, unfortunately such known start-up procedures involve extraordinary complexity in controlling various combustors, high-temperature blowers, complicated mechanical components, etc. Moreover, known solid oxide fuel cell power plants that include rapid -heating start-up systems invariably require non-plant based start-up power sources to operate the complicated rapid-heating start-up combustors, valves, control systems, etc, prior to production of any electricity by the plant.
Additionally, it is known that minimizing production of soot, or coking, during fuel reforming within the fuel processing system is not just a function of heating, but also typically includes provision of an adequate volume of high-temperature steam. An optimal steam to carbon ratio in such fuel processing systems is generally considered to be 2:1. A major challenge in producing an efficient start-up system for a solid oxide fuel cell power plant is to provide an adequate amount of steam at a sufficiently high-temperature so that when a carbon-based fuel is introduced into the fuel processing system, no coking occurs to contaminate fuel processing system and fuel cell components. Providing an appropriate amount of steam at a proper temperature to start-up a solid oxide fuel cell power plant from non-plant resources requires additional costs and sitting challenges that further reduce efficiencies of the power plant.
Known rapid-heating start-up systems of solid oxide fuel cell power plants have not provided efficient, soot-free start-ups, and therefore have not gained widespread usage due to their substantial costs to manufacture, install, operate and maintain. Therefore, there is a need for a solid oxide fuel cell power plant that includes an efficient, self-contained, rapid start-up system.